The present inention relates to a method of manufacturing semiconductor devices. More particularly the invention relates to a method of forming a flat surface on a semiconductor device.
Semiconductor integrated circuits have been highly integrated, as seen in a large scale integrated circuit, or LSI, or a very large scale integrated circuit, or VLSI. Keeping pace therewith, conductive layers on the semiconductor substrate are arranged in multiple layers. As a result, conductive layers and insulating layers are alternately formed one on the other. Thus, as the layers increase from two layers to three, for example, differences in the levels of layers increase.
If a difference in the levels of layers is large, it frequently tends to cause problems, difficulties or troubles such as, for example, disconnection or short-circuit of conductors. Methods of minimizing such difference have been considered, and one effective method developed is, after formation of a conductor layer according to a predetermined pattern, to apply liquid glass to cover the entire surface of th layer. The liquid gloss is then hardened, and a layer of phospho-silicate glass, or PSG, is caused to adhere to the surface of hardened liquid glass.
The aforementioned glass is a liquid of which the main component is silicic acid made by hydrolysis of methylsilicate, which is hardened when treated at high temperature to form an insulator. The main component of the insulator is silicon dioxide, as will be understood from the following: EQU Si(OCH.sub.3).sub.4 +4H.sub.2 O.fwdarw.Si(OH).sub.4 +CH.sub.3 OH
CH.sub.3 OH evaporates when heated and EQU Si(OH).sub.4 .fwdarw.SiO.sub.2 +2H.sub.2 O
in which H.sub.2 O also evaporates.
A film of silicon dioxide obtained by hardening of the liquid glass has a thermal expansion coefficient greatly different from that of the conductive layer of aluminum, for example, so that said film is susceptible to cracking where it is in contact with the conductor layer. If the thickness of the film is great, there are many possibilities of cracks.
If there are such cracks, when a window is opened in order to provide a connection of terminal pads, a liquid etchant used to open the window permeates into the cracks. it is difficult to remove such etchant. This results in inconveniences such as corrosion of the conductor layers and falling off of the PSG layer. This is a defect fatal to the reliability of the semiconductor device. Cracks are usually formed at the interface of the conductor layer and the film of oxidized silicon.
Although there are some variations due to the difference of viscosity, a liquid such as the aforementioned liquid glass generally tends to spread and adhere thinly on upper surfaces of protrusions such as conductor layers and thickly in dented places between the conductor layers. In spite of this, great care is taken to spread the liquid thin so that the glass layer has a thickness in the order of 500 to 2000 A to avoid formation of cracks. However, it is contrary to the anticipated object of maximum minimization of the difference of levels to so form a thin layer of liquid glass, and, as a matter of course, to do so has little effect in making the surface flat.
The principal object of the invention is to provide a method of manufacturing a semiconductor device which solves the problems and difficulties of the similar known methods.
An object of the invention is to provide a method of manufacturing a semiconductor device which minimizes the difference of levels in various layers formed on the surface of a semiconductor substrate by a method in which a hardened liquid glass is completely eliminated from the upper surface of a conductor layer, and the liquid glass is thickly formed in areas between the conductive layers.
Another object of the invention is to provide a method of manufacturing a semiconductor device which eliminates corrosion of conductors due to cracks formed when windows are opened to the conductors.
Still another object of the invention is to provide a method of manufacturing a semiconductor device which results in a much flatter surface of the semiconductor substrate than known methods.